The DC-DC converters are intended to convert a direct-current input voltage to a different direct-current output voltage and output the same to a load. The DC-DC converters are included in individual electronic circuits operating at different direct-current voltages in various electric products such as notebook personal computers to convert input voltages to stable direct-current voltages necessary for the electronic circuits and output the same. The DC-DC converters are divided by their operating principles into a non-insulated type that discontinues a current flowing into an inductor by a switching transistor and converts a direct-current input voltage to a direct-current output voltage different in voltage or polarity and an insulated type that increases or decreases an input voltage by a transformer. As a converter that converts a direct-current input voltage to an output voltage with a large potential difference from the direct-current input voltage, the insulated DC-DC converters are employed for battery chargers and AC adapters of portable electronic devices such as mobile phones and mobile music players.
FIG. 4 illustrates a flyback converter 100 as an example of the typical insulated DC-DC converters. In FIG. 4, reference sign 10a indicates a high-voltage terminal of a direct-current power supply 1, 10b a low-voltage terminal of the direct-current power supply 1, 11a a primary winding of a transformer 11, and 11b a secondary output winding of the transformer 11. A switching transistor Tr1 is connected in series with the primary winding 11a of the transformer 11 relative to the direct-current power supply 10, is composed of an FET (field-effect transistor), for example, and is controlled by a drive signal output from a drive circuit 3 to a gate of the switching transistor Tr1 to be opened or closed. While the switching transistor Tr1 is controlled to be closed (on-control) and is operating in a saturated state, an excitation current flows from the direct-current input power supply 10 to the primary winding 11a of the transformer 11. When the switching transistor Tr1 is controlled to be opened (off-control), the electric power accumulated in the transformer 11 by the excitation current flowing to the primary winding 11a during the period of the close control is released from the secondary output winding 11b. 
Provided on the secondary side of the transformer 11 are a rectifying diode 15 and a smoothing capacitor 16 constituting a rectifying and smoothing circuit that rectifies and smooths an output of the secondary output winding 11b, converts an input voltage Vin of the direct-current power supply 1 to an output voltage Vo between a high-voltage output line 20a and a low-voltage output line 20b, and outputs the same to a load connected between the high-voltage output line 20a and the low-voltage output line 20b. A voltage monitoring circuit 12 is provided between the pair of output lines 20a and 20b to monitor the output voltage Vo in comparison with a voltage set according to the rating of the load. The voltage monitoring circuit 12 on the secondary side of the transformer 11 and the drive circuit 3 on the primary side of the transformer 11 constituting a constant-voltage control circuit are connected by a photo coupler light-emitting element 13 and a photo coupler light-receiving element 14 performing photo-coupling.
The voltage monitoring circuit 12 controls the light emission of the photo coupler light-emitting element 13 as far as the output voltage Vo between the high-voltage output line 20a and the low-voltage output line 20b is over the set voltage. Upon receipt of the light emitted from the photo coupler light-emitting element 13, the photo coupler light-receiving element 14 outputs to the drive circuit 3 a limit signal indicating the state in which the output voltage Vo is over the set voltage. The output voltage Vo increasing or decreasing in accordance with the electric power accumulated in the transformer 11 can be controlled by increasing or decreasing the duration of the close control of the switching transistor Tr1 within a unit time. Accordingly, the drive circuit 3 performs variable control of on-duty of the drive signal by PWM modulation or PFM modulation with the limit signal. The drive circuit 3 decreases the on-duty of the drive signal for the close control of the switching transistor Tr1 while receiving the limit signal from the photo coupler light-receiving element 14, and increases the on-duty while not receiving the limit signal.
Accordingly, when the output voltage Vo is over the set voltage, for example, the drive circuit 3 outputs the drive signal with the decreased on-duty to the gate of the switching transistor Tr1, thereby to shorten the duration of the on-control within the unit time and lower the output voltage Vo. In contrast, when the output voltage Vo is lower than the set voltage, the drive signal with the increased on-duty is output to the gate of the switching transistor Tr1 to lengthen the duration of the on-control within the unit time and raise the output voltage Vo until the set voltage is exceeded. By repeating this process, the output voltage Vo is brought under constant-voltage control and kept at the predetermined set voltage.
In general, this kind of DC-DC converter may suffer a breakage or a fire of the load circuit when being brought into an unexpected abnormal operating state such as an overload or a short circuit of an output line. Accordingly, there is a DC-DC converter provided with a protection circuit to detect a drop in the output voltage, an abnormal rise in the output current, or the like, and shut off the output lines 20a and 20b (refer to Patent Literatures 1 and 2).
In addition, there is known a DC-DC converter that includes a protection circuit in which a fuse is connected in series with the primary winding 11a of the transformer 11 to shut off the current flowing to the primary winding 11a when the current exceeds the predetermined current rating.